Currently, cryptographic techniques are used to secure data during transmission electronically where the entire communication system is not physically secure. These techniques often involve complex keys and key holders to assure security, driving up the overall operating cost of this type of system. The most secure of the cryptographic units that are affordable, are limited in speed, running at a maximum data throughput of approximately 50.times.10.sup.6 bits/sec. (50 MBPS). This limitation is brought on because of computer overhead required by cryptographic techniques. To circumvent some of these problems Hughes Aircraft Co. has developed a secure fiber optic communication system that is based on the protection afforded by a guard mode. The guard mode carries a large amount of optical energy protecting a data carrying mode that has a relatively small amount of optical energy. When an intrusion is attempted, the light from the guard mode leaks out, an alarm trips, and the system shuts down. The Hughes system uses a costly special optical fiber to preserve the integrity of the alarm and data carrying modes. It is currently limited to operation at 13 MBPS over a maximum distance of 1.5 km. Also because of difficulties associated with mode integrity, the Hughes system is very difficult to implement where connectors must be used.
More recently, the Sagnac interferometer has been suggested as a means to support data communications using Faraday rotation (A Pseudo-Reciprocal Fiber-Optic Faraday Rotation Sensor: Current Measurement and Data Communication Applications, by P. Akhavan Leilabady, A. P. Wayte, M. Berwick, J. D. C. Jones, and D. A. Jackson, Optics Communications, Volume 59, Number 3, page 173-176, Sep. 1, 1986). This system uses twisted single mode optical fiber to reduce birefringence effects. However, it relies on toroidal current to generate magnetic fields and can be expected to operate at very low speeds of less than 1 MBPS. This system may also be susceptible to noise induced by stray magnetic fields, as one of its primary purposes is to measure current via the magnetic field induced Faraday effect.
Secure data communication links are needed that operate at high speed for everyday business, as well as government use. Banks transfer huge amounts of money by electronic means, usually computer to computer. They need means to assure that someone cannot intercept the data stream to change the recipient of the funds, change the amount transferred, or gain knowledge of who is transferring money where and to whom. Lawyers, accountants and securities brokers have need for absolute security in their corporate merger, acquisition, buyout and investment work to assure that advance information cannot be acquired by others before public notice, since acquisition of such information by an unscrupulous individual can result in fortunes being made or lost and liability to the lawyer, accountant or broker. Many businesses have proprietary information, such as financial data, costs, advanced product data that must be transferred electronically, where access by unauthorized people could result in ruin. Few, if any, persons outside of government can afford the slow, computer intensive, dedicated secure communication systems heretofore available. Therefore, there has been a need to provide economical secure data communication systems that can use existing fiber optic cables for high rate data transfer without resorting to encryption.